This invention relates generally to the accurate control of the flow of a gas into a vacuum facility. More specifically, it is directed to a method for monitoring the flow of a gas into a vacuum facility, which flow of a gas resulting from setting a desired flow of gas by means of at least one adjustable mass flow controller, which is interconnected between the vacuum facility and a pressurized reservoir arrangement for the gas.
Mass flow controllers are rather unstable regulating devices. They require to go regularly back to a calibration bench. For large companies which run many mass flow controllers the trend is to purchase and use internal calibration benches.
One of the most serious problems with mass flow controllers is that they tend to drift gradually, rather than jumping off a preset value, which latter would be easily detectable. Drift is critical to be permanently monitored, whereby it is further difficult to adequately set a limit to offset, which is still acceptable or is not anymore acceptable. With the drift of the mass flow controller, the properties resulting from any gas flow dependent process in a vacuum facility are likely to be drifting away, too. Thus, e.g. the properties of substrate or workpieces treated in a vacuum reactor will drift away due to mass flow controller drifting.
The reason why mass flow controllers, MFCs, can drift, is not fully known. One of the reasons is that MFCs use very narrow tubing and are therefore sensitive to perturbation by dust, even of very small grain. Another reason is related to the fact that MFCs are using heat transfer for sensing the flow. The hot area can promote locally chemical reactions and local chemical vapor deposition, especially when reactive gases are flown. Both these phenomena possibly at least contribute to unbalance of the heat sensing bridge and would explain that especially MFCs, which are used for flows of corrosive and reactive gases, such as for Ammonia, HCl, do drift as well as MFCs, which are used together with dust generating gases, as for Silane.
The consequences of pure MFC reliability is costing a lot to the industry. For instance, in semi-conductor manufacturing plants which are using hundreds of MFCs which are systematically exchanged for systematic control. Such operation is repeated every three to four weeks. The corresponding costs are considerable. Such frequent control does additionally induce secondary risks, such as gas line contamination due to frequent exposure to ambient of some sections of the gas lines during MFC exchange. However, despite the costs and the risks due to MFC systematic exchange, plant responsible staff have apparently concluded that these costs are still less serious than the risk and the consequences attached to drifting MFCs and drifting manufacturing processes.
The drift of MFCs is not easily predictable, some MFCs can remain perfectly stable for years, while another starts to drift from the very first day it is installed after calibration. Hence, the frequency for recalibration has to be defined very conservatively based rather on the worst case, and thus on a rather short lifetime of some MFCs. Thereby, the case where an MFC starts to drift the day after installation may clearly not be taken into account, but one must live with such risk.
In FIG. 1 there is shown prior art gas flowing into a vacuum facility, as especially into a vacuum treatment reactor.
A gas tank arrangement shown as a pressurized gas tank 1 is as an example pressurized to a pressure p.sub.1 of above 10 bar. Downstream of tank 1 there is provided a pressure regulator 2, which of reduces the input pressure p.sub.1 to an output pressure p.sub.2 slightly above 1 bar. Downstream of the regulator 2 there is provided a mass flow controller, MFC3. The pressure drop between the input and output of the mass flow controller 3 is about 1 bar, resulting in an output pressure p.sub.1 of about 0.01 bar.
Downstream of MFC3 there is provided the vacuum facility 4, as e.g. a vacuum surface treatment reactor, into which, as schematically shown, e.g. via a shower-like gas distributor, the gas from tank 1 is input.
The output pressure p.sub.3 at the output of MFC3 slowly drops along the connecting line to the reactor 4 according to its flow cross-section and length. The input nozzle arrangement causes a further slight pressure drop abutting in the process-required pressure p.sub.4 in the vacuum reactor. Via a throttle 5 the reactor 4 is connected to a vacuum pump 6.
Thus, when flowing all along from the gas tank 1 to the low-pressure vacuum facility 4 or process equipment, the gas experiences successive drops in pressure as is shown in this figure.
The gas is flowing naturally all the way to the pump 6 inlet due to the potential energy of the compression in tank 1 as delivered by the gas supplier, combined with the potential energy provided by the vacuum pump 6, keeping the pressure in the vacuum facility 4 on the desired low level.
The MFC regulates, accurately within the above mentioned limits, the gas flow as long as a predetermined pressure drop is maintained according to p.sub.23, which is e.g. at least 30 mbar. The minimum required pressure drop p.sub.23 is marked as p.sub.28min in FIG. 1. As the pressure p.sub.2 upstream the MFC is usually of the order of 1 to 2 bar, the MFC will be in good regulating operation as long as the output pressure p.sub.2 just after the MFC is in the order of 0.95 bar. There results that the pressure drop required across the MFC, p.sub.23min, is significantly lower than the actual pressure drop p.sub.23 according to FIG. 1, which is in the range of 1 bar. This is due to the fact that downstream of the MFC there prevails nearly the required vacuum facility low pressure p.sub.4, thus a process pressure in reactor 4, which may range, as an example, between 0.01 to 10 mbar or even much lower, i.e. down to the range of 10.sup.-3, 10.sup.-4 mbar.